1. (Field of the invention)
The present invention relates to a liquid-crystal display and, more particularly, to an active matrix display device using nonlinear devices each having two terminals.
2. (Prior arts)
When a large amount of information is displayed on a liquid-crystal display at a high density, the display is driven by the SBE method, the ferroelectric method, the active matrix method, or other method. In the active matrix method, either transistors or nonlinear two-terminal devices are formed as switches on a panel. In either case, voltage of a display signal is stored on each picture element active matrix. The method permits a large amount of information to be displayed with high image quality. Where, though, nonlinear, two-terminal devices are used as the switches, it is easier to fabricate a panel on which a large amount of information can be displayed at a high density, compared to using transistors, but it has been difficult to obtain high image quality. Two methods are available for driving an active matrix display device using such nonlinear, two-terminal devices as switches. One of them uses pulse-duration modulation, and is generally used to display gradation on a simple matrix display device. The other uses amplitude modulation that is employed to drive a panel where transistors are used as switches. Both of these methods for driving an active matrix display device using such nonlinear, two-terminal devices as switches are described below by referring to FIGS. 2 and 3.
FIG. 2(a) is a block diagram of a circuit for driving a display device by pulse-duration modulation. FIG. 2(b) is a block diagram of a column driver 8. FIG. 2(c) is a block diagram of a row driver 3. FIG. 2(d) is a waveform diagram of the voltage applied between row and column electrodes. The display device comprises an active matrix panel 1 of the nonlinear, two-terminal type, a row driver 3, a column driver 8, an analog-to-digital converter circuit 6, and a voltage source 7 for driving the liquid-crystal display.
The matrix panel 1 has row electrodes 1c, column electrodes 1d, and picture elements 10 arranged between the electrodes 1c and 1d. The picture elements 10 are arranged in rows and columns. Each picture element 10 consists of a nonlinear, two-terminal device 1a and a liquid-crystal layer 1b which are connected in series. Note that only one of the elements 10 is schematically shown. Normally, the panel is energized with 1 V, and the matrix is composed of 240 to 480 row electrodes k and 320 column electrodes 1d, for example. When color display is made, 320 column electrodes are provided for each of the three primary colors red, green, and blue. In that case, the number of picture elements would be 76,800 to 460,800.
As shown in FIG. 2(b), the column driver 8 for driving the column electrodes 1d comprises shift registers 8a, 4-bit latches 8b and 8c, a 4-bit coincidence circuit 8d, and a waveform synthesizer circuit 8e.
During the operation of the circuit, built as described above, an analog video signal is converted into digital form by the A/D converter circuit 6 and fed to the column driver 8. A timing signal is applied to one of the shift registers 8a of the driver 8 so that it may be clocked. The digital signal is stored in the corresponding 4-bit latch 8b. After data corresponding to about one line of information is stored in it, the data is transferred to another 4-bit latch 8c. Then, the coincidence circuit 8d compares the data held in the latch 8c with a 4-bit reference signal. The output signal is furnished to the waveform synthesizer circuit 8e that controls the time for which a selecting or non-selecting potential is applied to the column electrode 1d. In this case, the gradation is controlled with four bits and, therefore, 2.sup.4 =16 selecting periods are possible. The selecting and non-selecting potentials are supplied from the voltage source 7 as constant voltages of V.sub.0, V.sub.1, V.sub. 4, and V.sub.5. In order to drive the liquid-crystal display with alternating current, the waveform synthesizer circuit 8e switches the selecting potential to the non-selecting potential, or vice versa on every frame.
As shown in FIG. 2(c), the row driver for driving the row electrodes 1c comprises a shift register 3a and a waveform synthesizer circuit 3b. The output signal from the driver 3 is either a row-selecting signal or a row non-selecting signal applied to the row electrodes 1c. Their potentials are supplied from the voltage source 7 as V.sub.0, V.sub.1, V.sub.4, V.sub.5.
FIG. 2(d) shows the waveform of the voltage applied between one nonlinear two-terminal device 1a and the liquid-crystal layer 1b. This waveform is the difference between the output waveform from the column driver 8 and the output waveform from the row driver 3. Within a selecting period t.sub.s, a selecting voltage V.sub.s is maintained during a selecting pulse duration P.sub.s that assumes any one of 16 values, depending on the digitized video signal as already described. When the selecting voltage V.sub.s is applied to one picture element 10, the nonlinear, two-terminal device 1a contained in it exhibits a low resistance, charging the equivalent capacitance of the liquid-crystal layer 1b. At this time, the amount of charging can vary among 16 different values according to changes in the selecting pulse duration P.sub.s, whereby the gradation is controlled. During each non-selecting period t.sub.ns, a voltage of V.sub.ns is applied to each picture element 10. Under this condition, the nonlinear device 1a shows a relatively high resistance. Hence, the liquid-crystal layer 1b which was charged during the previous selecting period is gradually discharged. In this way, the gradation is controlled by pulse-duration modulation. This technique is disclosed in Patent Laid-Open No. 14890/1983.
The manner in which the gradation is controlled by amplitude modulation is described next. FIG. 3(a) is a block diagram of a display device using the amplitude modulation method. FIG. 3(b) is a block diagram of the column driver 2 shown in FIG. 3(a). FIG. 3(c) is a block diagram of the row driver 3 shown in FIG. 3(a). FIG. 3(d) is a waveform diagram of the voltage applied to each picture element 10 shown in FIG. 3(a).
Referring to FIG. 3(b), the column driver 2 comprises a shift register 2a and a set of analog switches 2b which are opened and closed by the output from the register 2a. The output signal from the register 2a varies according to every timing signal. The outputs of the analog switches 2b are connected with their respective column electrodes 1d. A video signal that is applied to the analog switches 2b is fed to the column electrodes 1d successively as information about voltage, via the analog switches which operate according to the timing signal. Thus, the video signal is sampled, and the sampled amplitudes are held. Then, they are fed to the column electrodes. It follows that the information about the voltage of the video signal, which was sampled and held, is retained by the capacity equal to the sum of the picture elements 10 on the same column electrode. Referring to FIG. 3(a), the video signal is supplied to an amplifier 9a having a gain of 1 and also to an amplifier 9b having a gain of -1 to produce signals having different polarities, for driving the liquid crystal with alternating current. The amplifiers 9a and 9b are included in the video amplifier circuit 9. A multiplexer 4 alternately supplies the output signals from the amplifiers 9a and 9b to the column driver 2 on every frame.
The row driver 3 shown in FIG. 3(c) is similar in circuit configuration and operation to one using pulse-duration modulation and, therefore, it will not be described below.
FIG. 3(d) is a diagram of waveforms applied to the picture element 10 shown in FIG. 3(a). The waveform shown in (i) shows the voltage of the video signal which was sampled and held on the column electrode 1d.
The waveform shown in (ii) shows the voltage applied to the row electrode 1c. In this waveform, P.sub.sel is a row-selecting signal. During this period, some of the row electrodes are selected. The selected row electrodes inject electric charges to the picture elements, or charges are released from the picture elements. That is, data is written and erased. P.sub.nsel shows a non-selecting signal. During this period, electric charges are injected to no picture elements 10, nor are charges released from any picture element. The condition set by the selecting period is maintained.
The waveform shown in (iii) shows the voltage applied to the picture element 10, and this is equal to the voltage difference between the waveform (i) applied to one column electrode and the waveform (ii) applied to one row electrode. When voltage V.sub.sel is applied to the picture element 10, the resistance of the nonlinear, two-terminal device 1a included in the element 10 sufficiently decreases. Under this condition, charges are injected to or released from the equivalent capacitance of the liquid-crystal layer 1b. During the non-selecting period t.sub.ns in which voltage V.sub.nsel is applied to the picture element 10, the resistance of the nonlinear, two-terminal device 1a included in the element 10 increases sufficiently, and so the electric charges which were injected during V.sub.sel are retained in the equivalent capacitance of the liquid-crystal layer 1b. At this time, the gradation of the picture element 10 is controlled according to the value of the selecting voltage V.sub.sel. The value of V.sub.sel depends on the voltage value of the video signal, which is sampled and held and applied to the column electrodes.
The aforementioned two driving methods have problems inherent in them.
In the pulse-duration method, it is possible to obtain somewhat high image quality, but this technique requires digital video data of more than 6 bits to permit an image to be reproduced in a natural way on a display device. To enable this, the number of drivers used per column electrode increases. As a result, the circuit for driving the column electrodes and the converter circuit for converting analog video signals becomes expensive.
When the amplitude modulation method is employed, the driving circuit is simplified, thus only a small number of elements are needed. Therefore, the cost is low. However, this method has the following disadvantages. As can be seen from the waveform (iii) of FIG. 3(d), the video signal which is sampled and held applies a signal having amplitude V.sub.nsel to one column electrode during each non-selecting period. This signal varies the resistance value of the nonlinear two-terminal device 1a of the picture element 10, causing shift of charges out of or into the equivalent capacitance of the liquid-crystal layer 1b. For this reason, the amount of charge which is written to or removed from the equivalent capacitance of the liquid-crystal layer 1b by the selecting voltage V.sub.sel for displaying purposes is also varied. As a result, one picture element is affected by the amplitude modulation of all other picture elements on the same column of electrodes, resulting in crosstalk along the column electrodes. Hence, sufficiently high contrast and image quality cannot be obtained.